Add dependency on protobuf.

[tswast]

This is required for the generated classes from the Models API.

Closes #7667

[tswast]

Tested locally with minimal virtualenv.

$ pip freeze
atomicwrites==1.3.0
attrs==19.1.0
cachetools==3.1.0
certifi==2019.3.9
chardet==3.0.4
google-api-core==1.8.2
google-auth==1.6.3
-e git+git@github.com:tsw...#egg=google_cloud_bigquery&subdirectory=bigquery
google-cloud-core==0.29.1
google-resumable-media==0.3.2
googleapis-common-protos==1.5.9
idna==2.8
mock==2.0.0
more-itertools==7.0.0
pbr==5.1.3
pluggy==0.9.0
protobuf==3.6.0
py==1.8.0
pyasn1==0.4.5
pyasn1-modules==0.2.4
pytest==4.4.0
pytz==2018.9
requests==2.21.0
rsa==4.0
six==1.12.0
urllib3==1.24.1



$ pytest tests/unit                                            
==================================== test session starts ====================================
platform darwin -- Python 3.6.4, pytest-4.4.0, py-1.8.0, pluggy-0.9.0
rootdir: /Users/swast/src/google-cloud-python/bigquery
collected 980 items                                                                         

tests/unit/test__helpers.py ......................................................... [  5%]
..................................................................                    [ 12%]
tests/unit/test__http.py ...                                                          [ 12%]
tests/unit/test_client.py ........................................................... [ 18%]
...................................................................................ss [ 27%]
s...                                                                                  [ 27%]
tests/unit/test_dataset.py .......................................................... [ 33%]
...........                                                                           [ 35%]
tests/unit/test_dbapi__helpers.py .....                                               [ 35%]
tests/unit/test_dbapi_connection.py ......                                            [ 36%]
tests/unit/test_dbapi_cursor.py ...................                                   [ 38%]
tests/unit/test_dbapi_types.py ...                                                    [ 38%]
tests/unit/test_external_config.py ...........                                        [ 39%]
tests/unit/test_job.py .............................................................. [ 45%]
..................................................................................... [ 54%]
..................................................................................... [ 63%]
........................sssss.................                                        [ 67%]
tests/unit/test_magics.py sssssssssssssssssss                                         [ 69%]
tests/unit/test_query.py ............................................................ [ 75%]
....................................                                                  [ 79%]
tests/unit/test_retry.py ..........                                                   [ 80%]
tests/unit/test_schema.py ...................................                         [ 84%]
tests/unit/test_table.py ............................................................ [ 90%]
..............................s....ss.sssssssssss....................s                [ 97%]
tests/unit/model/test_model.py ...............                                        [ 98%]
tests/unit/model/test_model_reference.py ..........                                   [100%]

========================== 938 passed, 42 skipped in 7.97 seconds ======================

Tested that it fails with older protobuf.

$ pip install protobuf==3.5.2
Collecting protobuf==3.5.2
  Downloading https://files.pythonhosted.org/packages/c4/7b/fbf783beb1c9d1766c64a9f4b4b80fd3f8732470697d01b3eb824d188486/protobuf-3.5.2-cp36-cp36m-macosx_10_6_intel.macosx_10_9_intel.macosx_10_9_x86_64.macosx_10_10_intel.macosx_10_10_x86_64.whl (1.2MB)
    100% |████████████████████████████████| 1.2MB 436kB/s 
Requirement already satisfied: six>=1.9 in /Users/swast/.pyenv/versions/3.6.4/envs/google-cloud-bigquery-minimal/lib/python3.6/site-packages (from protobuf==3.5.2)
Requirement already satisfied: setuptools in /Users/swast/.pyenv/versions/3.6.4/envs/google-cloud-bigquery-minimal/lib/python3.6/site-packages (from protobuf==3.5.2)
Installing collected packages: protobuf
  Found existing installation: protobuf 3.6.0
    Uninstalling protobuf-3.6.0:
      Successfully uninstalled protobuf-3.6.0
Successfully installed protobuf-3.5.2



$ pytest tests/unit          
==================================== test session starts ====================================
platform darwin -- Python 3.6.4, pytest-4.4.0, py-1.8.0, pluggy-0.9.0
rootdir: /Users/swast/src/google-cloud-python/bigquery
collected 302 items / 9 errors / 293 selected                                               

========================================== ERRORS ===========================================
________________________ ERROR collecting tests/unit/test_client.py _________________________
tests/unit/test_client.py:41: in <module>
    from google.cloud.bigquery.dataset import DatasetReference
google/cloud/bigquery/__init__.py:35: in <module>
    from google.cloud.bigquery.client import Client
google/cloud/bigquery/client.py:45: in <module>
    from google.cloud.bigquery.dataset import Dataset
google/cloud/bigquery/dataset.py:24: in <module>
    from google.cloud.bigquery.model import ModelReference
google/cloud/bigquery/model.py:27: in <module>
    from google.cloud.bigquery_v2 import types
google/cloud/bigquery_v2/__init__.py:23: in <module>
    from google.cloud.bigquery_v2 import types
google/cloud/bigquery_v2/types.py:22: in <module>
    from google.cloud.bigquery_v2.proto import model_pb2
google/cloud/bigquery_v2/proto/model_pb2.py:17: in <module>
    from google.cloud.bigquery_v2.proto import (
google/cloud/bigquery_v2/proto/model_reference_pb2.py:17: in <module>
    from google.api import annotations_pb2 as google_dot_api_dot_annotations__pb2
../../../.pyenv/versions/3.6.4/envs/google-cloud-bigquery-minimal/lib/python3.6/site-packages/google/api/annotations_pb2.py:15: in <module>
    from google.api import http_pb2 as google_dot_api_dot_http__pb2
../../../.pyenv/versions/3.6.4/envs/google-cloud-bigquery-minimal/lib/python3.6/site-packages/google/api/http_pb2.py:22: in <module>
    serialized_pb=_b('\n\x15google/api/http.proto\x12\ngoogle.api\"T\n\x04Http\x12#\n\x05rules\x18\x01 \x03(\x0b\x32\x14.google.api.HttpRule\x12\'\n\x1f\x66ully_decode_reserved_expansion\x18\x02 \x01(\x08\"\x81\x02\n\x08HttpRule\x12\x10\n\x08selector\x18\x01 \x01(\t\x12\r\n\x03get\x18\x02 \x01(\tH\x00\x12\r\n\x03put\x18\x03 \x01(\tH\x00\x12\x0e\n\x04post\x18\x04 \x01(\tH\x00\x12\x10\n\x06\x64\x65lete\x18\x05 \x01(\tH\x00\x12\x0f\n\x05patch\x18\x06 \x01(\tH\x00\x12/\n\x06\x63ustom\x18\x08 \x01(\x0b\x32\x1d.google.api.CustomHttpPatternH\x00\x12\x0c\n\x04\x62ody\x18\x07 \x01(\t\x12\x15\n\rresponse_body\x18\x0c \x01(\t\x12\x31\n\x13\x61\x64\x64itional_bindings\x18\x0b \x03(\x0b\x32\x14.google.api.HttpRuleB\t\n\x07pattern\"/\n\x11\x43ustomHttpPattern\x12\x0c\n\x04kind\x18\x01 \x01(\t\x12\x0c\n\x04path\x18\x02 \x01(\tBj\n\x0e\x63om.google.apiB\tHttpProtoP\x01ZAgoogle.golang.org/genproto/googleapis/api/annotations;annotations\xf8\x01\x01\xa2\x02\x04GAPIb\x06proto3')
E   TypeError: __new__() got an unexpected keyword argument 'serialized_options'
________________________ ERROR collecting tests/unit/test_dataset.py ________________________
...